1. Field of the Invention
The present invention relates to voltage monitoring technology for detecting decrease in the monitoring voltage and operating a protection circuit, and in particular, to an abnormal voltage detecting device capable of avoiding erroneous detection in a start up period of a voltage generating apparatus.
2. Description of the Related Art
Voltage generating apparatuses, typically power supply circuits and reference voltage generating circuits, are provided with a low voltage protection circuit for interrupting voltage output from the voltage generating apparatus when the output voltage has decreased below a predetermined lower limit voltage, as described in Patent Documents 1 and 2 (identified below). This low voltage protection circuit is provided in order to prevent a load circuit that operates while receiving the output voltage of the voltage generating apparatus from malfunctioning due to the decrease in the output voltage.
The abnormal decrease in the output voltage can be detected, for example, using a comparator CP having a construction shown in FIG. 4, which compares a monitoring voltage V1 corresponding to the output voltage with a reference voltage Vref that specifies the lower limit of the monitoring voltage V1. The monitoring voltage V1 can be obtained by resistance division of the output voltage and thus, is proportional to the output voltage. The comparator CP reverses the logical output thereof and delivers an abnormality detection signal Vout when the monitoring voltage V1 has lowered below the reference voltage Vref (V1<Vref), operating the low voltage protection circuit to interrupt the voltage output from the voltage generating apparatus.
The comparator CP of FIG. 4 comprises: a differential amplifier composed of a pair of transistors Q1 and Q2 with their loads of a pair of transistors Q3 and Q4 forming a current mirror circuit and driven by a first constant current source composed of a transistor Q5 coupled to a voltage VB; and a transistor Q7 that is driven by a second constant current source composed of a transistor Q6 and generates an abnormality detecting signal Vout based on receiving an output V2 of the differential amplifier.
However, the rise up of the monitoring voltage V1 of the voltage generating apparatus is not very fast in the startup period of the apparatus. As a result, the comparator CP may deliver an abnormality detection signal Vout, which would operate the low voltage protection circuit to stop voltage output of the voltage generating apparatus. As a consequence, the start up itself of the voltage generating apparatus may be disturbed.
To cope with this problem in a known low voltage protection circuit, the starting up situation is detected in the start up of the voltage generating apparatus using a reference voltage Vss for soft starting which can be a ramp voltage gradually increasing from the ground potential at zero volts. Based on the detection, reception of the abnormality detection signal Vout is inhibited until completion of the start up process of the voltage generating apparatus. After the completion of the start up process of the voltage generating apparatus is detected using the reference voltage Vss for soft starting in a monitoring device for monitoring abnormal decrease in the monitoring voltage V1, the output of the abnormality detection signal Vout is permitted for the low voltage protection circuit.
Specifically as shown in FIG. 5, in addition to a first comparator CP1 for comparing the monitoring voltage V1 with a reference voltage Vref, a second comparator CP2 is used for comparing the reference voltage Vss for soft starting with a start up determining voltage Vth for detecting completion of start up process of the voltage generating apparatus. The second comparator CP2 detects the completion of start up of the voltage generating apparatus and then, the gate circuit G is made active to permit the first comparator CP1 to deliver the abnormality detection signal Vout.
[Patent Document 1]
    Japanese Unexamined Patent Application Publication No. 2001-161062[Patent Document 2]    Japanese Utility Model Application Publication No. H06-002989
However, in order to detect abnormal decrease in the output voltage while monitoring start of the voltage generating apparatus using the reference voltage Vss for soft starting, an abnormal voltage detecting circuit of the voltage generating apparatus needs the two comparators CP1 and CP2 as described above and thus results in a complicated construction. In addition, since the above-described conventional construction renders the monitoring of the abnormal decrease in the output voltage inactive in the start up period of the voltage generating apparatus, it is impossible to know whether the voltage generating apparatus is working normally or not, which is another problem in the conventional technology.